Walnut rootstock ‘VX211’

ABSTRACT

A new and distinct variety of walnut rootstock denominated ‘VX211’ is described. This new variety, ‘VX211’, can be propagated through standard tissue culture micropropagation or rooted cuttings. ‘VX211’ has vigor and survivability in the nursery and in the orchard. It has reduced susceptibility to damage from nematodes ( Pratylenchus vulnus ) compared to other ‘Paradox’ rootstock. ‘VX211’ also has reduced susceptibility to damage from  Phytophthora citricola  in greenhouse screens and in the field compared to other ‘Paradox’ rootstock.

Latin name: Botanical/commercial classification: (Juglanshindsii×Juglans regia)/new ‘Paradox’ walnut rootstock.

Varietal denomination: The varietal denomination of the claimed walnutrootstock is ‘VX211’.

BACKGROUND OF INVENTION

The present invention relates to a new and distinct clonal rootstock of‘Paradox’ (not patented) walnut tree (Juglans hindsii×Juglans regia)that has been denominated varietally as ‘VX211’, and more particularlyto such a walnut rootstock that is vigorous, that has reducedsusceptibility to nematodes (Pratylenchus vulnus) and Phytophthora(Phytophthora citricola), and that further is easily clonally propagatedby standard tissue culture micropropagation.

It has long been recognized as desirable to provide vigorous walnutrootstocks that have increased resistance to soil pests and diseases,specifically nematodes (Pratylenchus vulnus) and Phytophthora(Phytophthora citricola). The rootstock of the present variety, ‘VX211’,is similar to other ‘Paradox’ walnut rootstocks (Juglans hindsii×Juglansregia), except that it has increased resistance or tolerance toPratylenchus vulnus and Phytophthora citricola and can be easilymicropropagated to produce a vigorous clonal ‘Paradox’ rootstock.

SUMMARY OF THE INVENTION

It was found that the walnut rootstock ‘VX211’ of the present inventionexhibits the following combination of characteristics:

-   -   a) can be propagated through standard tissue culture        micropropagation or rooted cuttings;    -   b) has vigor and survivability in the nursery and in the        orchard;    -   c) has reduced susceptibility to damage from nematodes        (Pratylenchus vulnus) compared to other ‘Paradox’ rootstock; an    -   d) has reduced susceptibility to damage from Phytophthora        citricola in greenhouse screens and in the field compared to        other ‘Paradox’ rootstock.

BRIEF DESCRIPTION OF THE TABLES

Table 1 shows comparative nursery performance of ‘VX211’ and otherwalnut rootstock clones.

Table 2 shows growth rating and diameters for ‘VX211’ and otherrootstock clones.

Table 3 shows field performance of clonal ‘Paradox’ walnut hybrids,Northern California black walnut and Chinese wingnut rootstocks innon-infested soil and soil infested with Phytophthora.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows performance of ‘VX211’ compared to ‘AX1’ under pressurefrom nematodes at varying densities.

FIG. 2 shows a visual rating of tree growth and condition of clonal andseedling test trees at a Jenny Lind, Calif. field site.

FIG. 3 shows cumulative percent mortality of clonal and seedling testtrees at a Jenny Lind, Calif. field site.

FIG. 4 shows tree mortality of clonal and seedling test trees at aLinden, Calif. field site.

FIG. 5 shows relative resistance to Phytophthora citricola among clonaland seedling test trees in greenhouse experiment.

FIG. 6 shows ‘VX211’ at 10 years of age.

FIG. 7 shows bark of ten-year old ‘VX211’.

FIG. 8 shows several three-year old trees of ‘VX211’.

FIG. 9 shows bark at base of three-year old ‘VX211’.

FIG. 10 shows bark and lenticels farther up the stem of ‘VX211’.

FIG. 11 shows 6-month old greenhouse grown ‘VX211’.

FIG. 12 shows grafted ‘VX211’.

FIG. 13 shows spring foliage of ‘VX211’.

FIG. 14 shows deep red of new foliage of ‘VX211’.

FIG. 15 shows upper side of mature leaf of ‘VX211’.

FIG. 16 shows lower side of mature leaf of ‘VX211’.

DETAILED DESCRIPTION OF THE INVENTION

The new rootstock of the present invention was selected as part of the“Paradox Diversity Study” (PDS) which was initiated in 1996 to study thegenetic diversity of commercial ‘Paradox’ sources. The study includedapproximately 300–500 seeds each (depending on the predicted percent‘Paradox’), from 37 black walnut sources of ‘Paradox’ provided byCalifornia walnut nurseries, and seven controlled crosses made in Davis,Calif. and open-pollinated controls from different Juglans species.Seeds were germinated and grown at 3 different nurseries for one yearand then seedlings were distributed to cooperating researchers for testsof response to nematodes, Phytophthora (seed supplied), crown gall(Agrobacterium tumefaciens) and the field environment (field trials).The study was repeated in 1997.

The rootstock of the present invention was evaluated for response tonematodes in 1998 along with 9 siblings and the remaining rootstockfamilies. One-year old seedlings were planted on 1.2 or 1.8 meterspacing with 3.35 meter centers. The field test site was infested with asingle population of root lesion nematode (P. vulnus) originally placedon site in 1976. At planting time seedlings were inoculated withadditional P. vulnus. Each fall 20 grams of root tissue were collectedfrom each tree. These roots were placed in a mist chamber for 5 days fornematode extraction and nematodes/gram root was calculated.

In July 1998 it was evident that one seedling (‘VX211’) was morevigorous than the others, but in the fall the nematodes were abundant onthe roots of ‘VX211’. In July 1999 roots were collected and againnematodes were found to be abundant, but the seedling ‘VX211’ continuedto be more vigorous than the other seedlings in spite of the nematodes.Due to its apparent superiority it was transplanted to a “mother block”at an agriculture center in Parlier, Calif. In winter 2000–2001propagating wood was collected. ‘VX211’ was propagated by hardwoodcuttings. Additionally, ‘VX211’ was asexually reproduced by standardtissue culture micropropagation in Davis, Calif. In 2002 a “stock block”was established in Davis, Calif. with 6 trees of ‘VX211’ as well asother promising selections.

Propagation of ‘VX211’ both by standard hardwood cuttings and bystandard tissue culture micropropagation was successful. From November2002 to October 2003, 212 ‘VX211’ plants were micropropagated, rooted ingelled medium and acclimatized in the greenhouse; 153 (72%) survived.When rooted ex vitro, 126/184 (68%) survived. Hardwood cuttings rootedbetween 73% (11/15) to 87% (13/15). By November 2003 there were 187available for field trials and 60 available for Phytophthora screening.

In March 2004, 48 plants of the ‘VX211’ clone and a standard ‘Paradox’(‘AX1’) were evaluated for additional nematode screening and comparison.These were planted in 1/100^(th) acre macroplots. The macroplots hadconcrete sides 1.5 meter deep into the soil with open bottoms and werenematode-free. ‘VX211’ and ‘AX1’ were planted side by side in 48separate macroplots infested with 0, 1, 20, or 500 P. vulnus nematodesper 250 cc of soil. Tree diameters and number of nematodes on the rootswere determined 2004–2006 (FIG. 1). Nematodes built up quickly but‘VX211’ was 30% taller in the first year and the diameter of ‘VX211’ wassignificantly greater than ‘AX1’ in all three years (FIG. 1). The vigorof ‘VX211’ under pressure from nematodes suggests that ‘VX211’ has ameans to avoid or escape severe damage from nematodes.

In spring 2004, 106 plants of ‘VX211’ produced through standard tissueculture micropropagation were planted in a nursery along with over 1800plants of 17 different clones. At the end of the growing season ‘VX211’was the most vigorous of all clones (Table 1) demonstrating thatpropagation is true-to-type through successive generations. Eighty-twopercent were graftable and the mean diameter was 31 mm at 5 cm from thesoil surface.

Graftable trees were distributed for grafted field trials in 5 differentorchards in replant situations in 2005. Grafting posed no problem and‘VX211’ is considered compatible with English walnut scions. It is atypical ‘Paradox’ in that respect. In addition, 30 each of 11 differentgenoptypes including ‘VX211’ were planted in May 2005 in Davis, Calif.for artificial inoculation with Phytophthora citricola. A randomizedblock split plot design was used. For each rootstock clone, there weresix four-tree plots to be infested and six single-tree plots to serve asnon-infested controls. Northern California black (Juglans hindsii) andwingnut (Pterocarya stenoptera) were included as susceptible andresistant controls, respectively. In January 2006, 100 ml of a V8juice-oat mixture infested with Phytophthora citricola was mixed intothe upper 5 cm of soil around the trunk of each tree. A sterile mixturewas applied to the uninoculated controls.

Early results from several of the grafted field trials are shown (FIGS.2–4; Table 2). In all cases ‘VX211’ was one of the superior clones. Theblock artificially inoculated with Phytophthora was assessed for growthin trunk circumference and development of crown rot as indicated bytrunk cankers extending up from the soil surface in November 2006.Sixty-two percent of the susceptible controls were rotted or dead. Theuninoculated controls of ‘VX211’ were the most vigorous trees in theblock apart from the wingnut controls. No cankers were found on ‘VX211’or many of the other clones, however the Phytophthora inoculation didappear to depress growth somewhat in ‘VX211’ (Table 3).

Simultaneously with field trials, greenhouse trials were carried out toassess the relative susceptibility of ‘VX211’ and other selected clonesto Phytophthora citricola. Standard Phytophthora screening methods wereused. Clonal selections including ‘VX211’ were micropropagated, rooted,acclimatized and chilled, and at 2–6 months were transplanted to pots ofartificially inoculated soil. Four isolates of P. citricola fromdifferent districts of California were used to infest the soil. Theisolates were grown in separate jars of V8 juice-oat-vermiculitesubstrate for one month, mixed in equal proportions and mixed in thesoil (40 ml inoculated substrate per liter of soil). Starting two weeksafter transplanting, all plants received 48-hour periods of soilflooding every two weeks. Three months after transplanting, soil waswashed from the plants and the incidence and severity of crown rot weredetermined. One selected clone, ‘VX211’ consistently showed moderateresistance. The results from 2006 trials are shown in FIG. 5.

BOTANICAL DESCRIPTION OF THE PLANT

This description is based on the original selection of ‘VX211’,ungrafted, at ten years of age, a 3-year old ungrafted tree inPhytophthora field screen, and a greenhouse-grown plant at 6 months ofage. Figures are also shown of a grafted ‘VX211’ tree. Data for thebotanical description were collected in spring 2007.

The Munsell Color Charts for Plant Tissues (1977. GretagMacbeth, NewWindsor N.Y.) is used in the identification of color. Also, common colorterms are to be accorded their ordinary dictionary significance,

-   Botanical classification: Juglans hindsii×Juglans regia.-   Female parent: Juglans hindsii.-   Male parent: Juglans regia.

‘VX211’ differs from its female parent by having fewer leaflets/leaf,broader leaflets and hybrid vigor. ‘VX211’ differs from its male parentby having more leaflets/leaf and hybrid vigor. ‘VX211’ does not differsubstantially from other similar hybrids except in its superiorperformance under adverse soil conditions as described in the“Background of the Invention”.

-   Plant: The growth habit of the tree is illustrated in FIG. 6. This    10 year old tree is approximately 7.3 meters in height with a canopy    diameter of approximately 5 meters. The trunk circumference at 1.2    meters above ground level is about 0.61 meters. The bark and    year-old branches are light brown (2.5Y 7/2) (FIG. 7). New shoots    are green (5GY 7/6). Lenticels (approximately 12 per 2.5 square cm)    are slightly lighter than the bark (2.5Y 8/2). The 3 year old trees    are 4.3–4.9 meters tall (FIG. 8). The bark is brownish-green (2.5GY    5/8) with scattered (22/2.5 square cm.) buff-colored lenticels    (7.5YR 8/2) (FIGS. 9, 10). The six month old greenhouse grown tree    is about 35 cm. tall and the main stem is about 1 cm. in diameter    (FIG. 11) and green (5GY/10). Lenticels about 0.5 mm long are more    dense at the base of the plant and are a buff color (2.5Y 8/4).    Graft take is in the normal range for seedling ‘Paradox’ walnuts    (FIG. 12).-   Foliage: The slightly pubescent new spring foliage has a reddish hue    to it (10R 4/8), darkest towards the tip (FIGS. 13 and 14) turning    green (5GY 5/6) as the leaves get older. The leaves are smooth and    the margins are entire (not serrate). The leaves are pinnately    compound with 13–15 leaflets. The mature leaves of the 6 month old    plant have 9–11 leaflets and are 30 cm long and 23 cm wide. The    number of leaflets may vary depending on the age and size of the    plant. The upper leaf surface is bright green and the same color as    the stem (5GY 5/10) (FIG. 14). The lower surface is slightly duller    (5GY 6/6) (FIG. 16). The leaflets are about 5 cm wide and 14 cm long    with a petiole 4–8 cm long.-   Inflorescence: No catkins or female flowers appeared in the first 10    years. The tree is probably male sterile as is typical with Juglans    hindsii×Juglans regia hybrids. No nuts were observed.-   Disease resistance and susceptibility: This rootstock is typical of    other Juglans hindsii×Juglans regia hybrids except that it possesses    higher vigor and ability to survive heavy nematode loads. It is also    less susceptible to Phytophthora citricola than other similar    hybrids.-   Usage: The new rootstock of the present invention provides walnut    growers with a new clonally propagated ‘Paradox’ rootstock. It can    be easily micropropagated.

TABLE 1 Clones grown in Stanislaus County, California in 2004 Graft-Graft- Planted able able Diameter (mm) Clone N N % Mean SD Range CVNematodes VX211 106 87 82 31 4.9 21-44 12.6 Phytophthora AZ2 230 151 6626 5 13-38 19.2 AZ3 49 24 49 25 6.7 11-37 26.8 NZ1 172 111 64 26 4.410-39 16.9 JX2 246 191 78 29 4.1 13-39 14.1 RX1 104 78 75 18 1.6 14-228.8 AX1 163 86 53 27 4.3 14-40 15.9 GZ1 108 83 77 26 5.4 13-40 20.8 Px1247 154 62 26 4.6 12-40 17.7 AZ1 52 38 73 30 4.4 22-43 14.7 UX1 27 23 8525 4 15-30 16 GZ2 47 38 81 26 4.5 15-33 17.3 Blackline WIP3 158 66 42 265 12-35 19.2 WIP2 10 6 60 25 2.3 23-99 9.2 Control UX022 71 59 83 23 3.714-29 16.1 English Vina 14 10 71 18 3.7 13-24 20.5 Sunland 64 20 31 263.8 18-31 14.6 Totals 1868 1225 66 25

TABLE 2 Walnut Rootstock Orchard Trials-Yuba County. Established May2005. Randomized complete block design with 4 clonal and 1 seedling‘Paradox’ rootstock. Twelve single tree replicates. All rootstocksgenerally grew well except AZ2. Midday stem water potential measurementssuggest that replants and adjacent orchard trees were generally in asimilar range to the replants. AZ2 did not grow well despite havingmidday stem water potentials in a similar range to adjacent trees andthe other replants. There was 100% survival of all replant sources. JuneDecember November % Change 2005 June 2005 2005 2006 (June 2005 GrowthDiameter Diameter Diameter to November Treatment Rating* (mm) (mm) (mm)2006) VX211 3.0 29.0 31.2 42.5 a +46.6 AZ2 2.0 26.9 28.5 26.2 c −2.6 NZ12.9 23.5 25.8 35.6 b +51.4 JX2 2.2 23.7 26.4 35.2 b +48.5 CONTROL 1.420.5 22.4   30.0 bc +46.3 *rootstocks were headed near or below 2005diameter measurement point. 2006 measurement is on new shoot growingabove previous cut. *Growth rating descriptions: 0 No growth 1 Justbreaking 2 Moderate growth 3 Vigorous growth

TABLE 3 Field performance of clonal ‘Paradox’ walnut hybrids, NorthernCalifornia black walnut and Chinese wingnut rootstocks in non-infestedsoil and soil infested with Phytophthora. Clone Maternal background (orof hybrid (or Soil treatment Incidence of species) species of standard)(January 2006) crown rot (%) AX1 californica Control 0 c P. citricola 4c AZ2 (major x hindsii)x nigra Control 0 c P. citricola 0 c NZ1 (major xhindsii)x nigra Control 0 c P. citricola 0 c GZ1 hindsii Control 0 c P.citricola 4 c JX2 hindsii Control 0 c P. citricola 0 c PX1 hindsiiControl 0 c P. citricola 8 bc VX211 hindsii Control 0 c P. citricola 0 cRX1 microcarpa Control 0 c P. citricola 0 c WIP3 hindsii x regia Control0 c P. citricola 8 bc (NCB) (J. hindsii) Control 16 b P. citricola 62 a(Wing- (Pt. stenopiera) Control 0 c nut) P. citricola 0 c Clone Percentof Incidence of Increase in (or trunk circ. tree mortality trunk circ.species) Necrotic (%) (mm) AX1 0 c 0 c 163 c 1 c 0 c 146 cde AZ2 0 c 0 c116 fg 0 c 0 c 117 fg NZ1 0 c 0 c 116 fg 0 c 0 c 130 def GZ1 0 c 0 c 157cd 1 c 0 c 150 cd JX2 0 c 0 c 166 bc 0 c 0 c 135 def PX1 0 c 0 c 169 bc1 c 0 c 157 cd VX211 0 c 0 c 191 b 0 c 0 c 147 cde RX1 0 c 0 c 112 fg 0c 0 c 116 fg WIP3 0 c 0 c 100 g 2 c 0 c 121 efg (NCB) 17 b 17 b 65 h 59a 59 a 57 h (Wingnut) 0 b 0 c 226 a 0 b 0 c 193 b ¹All trees wereplanted May 2005. The assessments of crown rot and mortality were made21 Nov. 2006. Means within a column and without letters in common aresignificantly different (Waller k ratio).

1. A new and distinct variety of walnut rootstock plant designated‘VX211’ as shown and described herein.